OK, that's kinda silly. If a 30HD creature has a THAC0 of -1, and my character has an AC of -8, I still want the DM to make a roll! It still needs a 7 to hit!!!!

If I understand Gygax's wording (and I'm not sure I do), the 30-HD monster in your example would indeed need a 7, but if the target's AC were 10 instead, the same monster would need -11 (an automatic hit), and this -11 would become +11 damage.